In melting a glass raw material, a so-called refining agent is added. A refining agent has, in general, the effect of growing bubbles by a gas generation through its decomposition and thereby promoting the removal of the bubbles contained in the molten glass in the initial stage of the raw material melting process. In the later stage of the melting process, the refining agent also has an effect of absorbing the gas in residual minute bubbles and promoting the elimination thereof.
In the case of an alkali-free glass, arsenic trioxide (As2O3) has been used as such a refining agent. The refining function of As2O3 is attributable to the effect that As2O3 added to the raw material takes in oxygen from the surroundings in the temperature raising process so that it turns into As2O5, and then As2O5 releases the oxygen when it turns back to As2O3 again at a higher temperature. Therefore, when using arsenic trioxide as a refining agent, an oxidizing agent is added into the glass raw material in order to accelerate the change from As2O3 to As2O5 in the initial stage of the melting reaction of the material. As such an oxidizing agent, a nitrate commonly is used.
Meanwhile, it is known that antimony trioxide (Sb2O3) shows the same effect as As2O3. “Handbook of Glass Engineering” (edited by Masayuki Yamane, et al., Asakura Shoten, 1999, p. 292) describes that antimony trioxide (Sb2O3) is “less toxic than arsenic trioxide, and therefore has more often been used instead of arsenic trioxide.”
Recently, in various fields, there have been concerns about the adverse impacts of the use of toxic substances on the environment. Also in the field of an alkali-free glass mainly used for TFT liquid crystal display substrates, the replacement of As2O3, which has conventionally been used as a refining agent, with more environment-friendly Sb2O3 has been proposed (see, for example, JP10-114538A and JP10-231139A).
JP10-114538A discloses an alkali-free glass production method in which As2O3 is not used as a refining agent, characterized in that “0.05 to 2 wt % of SnO2 and 0.05 to 3 wt % of Sb2O3 are added as refining agents.”
JP10-231139A discloses an alkali-free glass production method in which As2O3 is not used as a refining agent, characterized in that “0.05 to 3 wt % of Sb2O3 and 0.01 to 2 wt % of a chloride in terms of Cl2 are added as refining agents.”
A compound containing pentavalent antimony sometimes is used as a refining agent, not so often as antimony trioxide (Sb2O3) that is a trivalent antimony oxide. “Handbook of Glass Engineering” (edited by Taro Moriya, et al., Asakura Shoten, 1963, p. 298) describes that “sodium antimonate (Na2O.Sb2O5.1/2H2O) or sodium orthoantimonate (Na3SbO4) have a refining effect.”
JP11-35338A discloses “an antimony-based refining agent that is a multiple oxide of pentavalent Sb and at least one element selected from the group consisting of Mg, Zn, Ca, Sr, Ba, Li, K, Al, Si, Ti, Sn, Zr, Ce, La, Nb and P.”
JP11-49520A discloses “a glass melting method in which antimony pentoxide is used as a refining agent.” In this publication, it is pointed out that when antimony pentoxide is used instead of antimony trioxide as a refining agent, a nitrate that is an oxidizing agent need not be added to change antimony trioxide into antimony pentoxide, and thus if the nitrate need not be added, melting segregation of a raw material caused by the addition of a large amount of nitrate can be reduced (paragraph 0012).
JP11-49520A describes in its Examples the numbers of bubbles measured when antimony pentoxide was added to raw glass batches. Referring to this section, when 0.5 parts by mass of antimony pentoxide was added per 100 parts by mass of the raw glass batch (in terms of an oxide), the number of bubbles measured in the case where a nitrate was not added (Example 3) was less than that measured in the case where the nitrate was added (Example 2) (Tables 2 and 3). Also from the viewpoint of melting segregation, the case where the nitrate was not added (Example 3) was superior to the case where the nitrate was added (Example 2) (Table 4). In view of the above, in the method disclosed by this publication, a nitrate should not be added when antimony pentoxide is used in a raw material (Claim 2).
JP2001-328820A discloses “a glass production method in which a batch of a cullet containing no arsenic oxide and a refining agent other than arsenic oxide are melted to be formed.” Examples of a refining agent other than arsenic oxide include Sb2O5 and/or SnO2.
In the methods described in JP10-114538A, JP10-231139A and JP11-49520A, there is still room for improvement to achieve the required quality of bubbles in the case of a glass that is melted at a high temperature like an alkali-free glass.
The “antimony-based refining agent” described in JP11-35338A has a problem in that it is more expensive than antimony trioxide (Sb2O3) and antimony pentoxide (Sb2O5), or that it cannot be used depending on its composition.
In the method described in JP2001-328820A, there is a problem that a sufficient refining effect cannot be obtained in the case of melting a common mixture of glass raw materials with less content of cullet.